This invention relates to cutting elements and more specifically to high energy shear cutting elements having a cutting layer comprising polycrystalline cubic boron nitride (PCBN) and to bits incorporating the same and more specifically to turbine driven bits incorporating the same.
Shear cutting elements used in earth-boring bits have a tungsten carbide substrate body over which is bonded a polycrystalline diamond (“PCD”) material cutting layer which is used to do the cutting. PCD materials, however, have many limitations. For example, the thermal stability of PCD deteriorates due to graphitization at temperatures above 650° C. This is typically caused by the air and the presence of cobalt around the diamond crystals. Similarly, cracking tends to develop at temperatures above 650° C. due to thermal mismatch between the diamond crystals and cobalt forming the PCD material layer.
In an effort to get PCD to operate at higher temperatures, the catalyst found in PCD material is sometimes fully leached so as to enhance the thermal stability of the PCD material. In such case, the PCD can withstand temperatures up to 1000° C. in air. However, graphitization of the PCD does increase with temperatures above 1000° C. Consequently, shear cutting elements having a PCD cutting layer cannot be used to cut earth formation when the cutting layer temperature will exceed 650° C. or 1000° C., depending on whether the PCD layer is leached.
Cutting layer temperatures of cutting elements on bits driven by high speed turbines or motors (hereinafter “turbine bits”) exceed 1000° C. These bits are operated at greater than 300 RPM, to cut formations having a strength greater than 10,000 psi. Turbines driving turbine bits typically operate at RPM levels of 1,000 RPM or greater and the turbine bits rotate at levels of about 300 RPM or greater.
PCBN has been disclosed for use in cutting elements for drilling of formations. However, to date no PCBN materials is used as a cutting layer in earth-boring conditions due to its relatively poor strength and toughness when compared to PCD.